Sol solution and method for film formation

ABSTRACT

A method for forming a protective layer for a dielectric material in an alternating current type plasma display is provided. In this connection, a coating sol solution is provided which can form a protective layer on a large area substrate without the need to introduce any expensive equipment, the protective layer thus formed being excellent in properties such as strength, adhesion, transparency, and protective properties and capable of being formed by a sol-gel process without use of the conventional vacuum process. The sol solution comprises a dispersion of a precursor to magnesium oxide in a specific form. A method for film formation, using this solution is also provided.

This is a division of application Ser. No. 08/726,173 filed Oct. 4,1996, now U.S. Pat. No. 6,149,967.

BACKGROUND OF THE INVENTION

The present invention relates to a sol solution and a method for filmformation, and more particularly to a sol solution useful for theformation of a protective layer for a dielectric layer in an alternatingcurrent type plasma display and a method for film formation using thesol solution.

For optical components, electronic and electrical components, magneticmaterial components and the like, layers having various functions areformed on the substrate, and, if necessary, a protective film isprovided on the surface of these layers for protection purposes. Highfilm strength and large adhesion to the above functional layers aregeneral properties required of such protective films.

In recent years, research and development of flat panel displays as analternative to CRT have been energetically conducted. Among them, theso-called “plasma display” which utilizes luminous phenomenonaccompanying discharge in a display is classified, according to thestructure of the electrode (mainly ITO), roughly into a direct currenttype, wherein metallic electrodes are exposed to a discharge space, andan alternating current type wherein electrodes are covered with adielectric layer. In the latter alternating current type plasma display,those produced by both a thin-film process using a vacuum system and athick-film process using screen printing have begun to be put topractical use.

When the use of the plasma display in a color television with a largescreen size is contemplated, the plasma display should have a memoryfunction from the viewpoint of increasing the brightness. In thisrespect, the alternating current type plasma display inherently has amemory function by virtue of charges accumulated in the protective layerprovided on the dielectric layer and, hence, is considered to be able tocope with a demand for an increase in screen size. Magnesium oxidehaving high secondary electron emission efficiency and excellentsputtering property has been used as a material for the protectivelayer. At the present time, a service life of 15000 hr. has beenachieved for a full-color, alternating current type plasma display, anda panel having a diagonal distance of 21 in. has been put on the market.

Methods for forming the protective layer include thin-film processes,such as EB deposition, sputtering, and CVD (Japanese Patent PublicationNos. 42579/1985 and 59221/1988), and thick-film processes, such as onethat comprises spray-coating basic magnesium carbonate as a startingmaterial for magnesium oxide on a substrate to form a thick coating andfiring the coating to convert the basic magnesium carbonate to a metaloxide (Japanese Patent Publication No. 13983/1982) and one thatcomprises dispersing a fine powder of magnesium oxide in a liquid binderwhich, upon firing, can be converted to an oxide, thereby forming amagnesium oxide film (Japanese Patent Publication No. 283020/1994).

Among the above methods, those using a vacuum process, such as EBdeposition, sputtering, and CVD, are disadvantageous in that it isdifficult to accommodate a large panel substrate, like a plasma display,in a vacuum chamber, posing problems of cost of equipment andproductivity when an increased screen size is assumed.

Coating is a simple method and, hence, has been extensively andintensively studied in the art. At the present time, however, nosatisfactory performance could have been attained yet. The reason forthis unsatisfactory performance is as follows. In the coating, amagnesium oxide printing paste containing magnesium oxide particles isused for printing a magnesium oxide protective layer for alternativecurrent type PDP. In order that the protective layer has sputteringresistance, magnesium oxide should be in the form of homogeneousparticles having a diameter of 30 to 300 nm and, at the same time, theparticles should be homogeneously dispersed in a binder. However, fineparticles of magnesium oxide are likely to agglomerate and becomes verydifficult to be homogeneously dispersed in the binder. For this reason,in the conventional commercially available printing paste, the diameterof incorporated magnesium oxide particles per se is large and theviscosity of the paste per se is high, making it difficult to reduce thethickness of the protective layer. Consequently, no highly evenprotective film can be formed. Further, the conventional printing pasteis disadvantageous in that the conventional heat treatment process (600°C. or below) cannot offer satisfactory film strength, adhesion and otherproperties and heat treatment causes cracking of the film. Thecoatability of the paste is also unsatisfactory. Furthermore, themagnesium oxide particles incorporated are large, and the viscosity ofthe paste per se is high, making it difficult to reduce the layerthickness. This in turn poses a problem that a demand for minimizedfiring voltage and drive voltage cannot be met.

In order to solve the above problems involved in the coating method, thepresent inventors have previously proposed a method, using the so-calledsol-gel process, that comprises the steps of: coating a magnesiumhydroxide sol, prepared by hydrolyzing a magnesium oxide compound, ontoa substrate; drying and firing the coating to form a magnesiumoxide-containing thin film (Japanese Patent Laid-Open No. 111177/1996).

This method, however, is disadvantageous in that the adhesion of themagnesium hydroxide sol to the substrate is so poor that a part of theresultant magnesium oxide layer is easily separated from the substrateand the film thickness is uneven. Therefore, the above method issubstantially ineffective in improving the firing voltage and the drivevoltage (power consumption). Further, heat treatment of the magnesiumhydroxide sol at a conventional temperature (600° C. or below) does notaccelerate the crystallization of the resultant magnesium oxide.Therefore, the film thus obtained has a low crystallinity and, hence,unsatisfactory film strength, causing a problem that properties as aprotective film cannot be satisfactorily exhibited.

On the other hand, the use of an organometallic compound, such as amagnesium alkoxide, as a starting compound for the magnesium hydroxidesol can offer magnesium oxide having relatively good adhesion to thesubstrate and uniformity in the film. Magnesium alkoxides and the likeinvolve a problem of cost, because they are very expensive, and, inaddition, a problem associated with handling because the reactivity ofthe magnesium alkoxides is so high that the control of the reaction isdifficult and the life of the resultant magnesium hydroxide is short.

Under the above circumstances, the present invention has been made, andan object of the present invention is to solve the above problems of theprior art and to provide a sol solution that can form a film on a panelhaving a large area without the need to use expensive equipment such asrequired in the vacuum process, the formed film having excellent filmstrength, adhesion, transparent, protective properties and otherproperties and being usable as a protective layer that can lower thefiring voltage and the drive voltage (power consumption), and to providea method for film formation using said sol solution.

DISCLOSURE OF INVENTION

The first invention relates to a sol solution comprising a dispersion offine particles of magnesium hydroxide bonded to a polyhydric alcohol ora derivative thereof in an organic solvent containing an organiccompound having at least one hydroxyl group, and a method for filmformation using said sol solution.

The second invention relates to a sol solution comprising a dispersionof an agglomerate of fine particles of magnesium hydroxide in an organicsolvent containing at least one hydroxyl-containing organic compound,and a method for film formation using said sol solution.

The third invention relates to a sol solution comprising a dispersion ofa partial or complete hydrate of magnesium acetate tetrahydrate ascolloidal particles in a medium composed mainly of water, and a methodfor film formation using said sol solution.

According to the present invention, the formation of a protective layer,for example, in an alternating current type plasma display from theabove specific sol solution realizes the formation of a film, as theprotective layer, on a panel having a large area without the need to useexpensive equipment such as required in the vacuum process, the formedfilm having excellent film strength, adhesion, transparent, protectiveproperties and other film properties and being usable as a protectivelayer to provide an alternating current type plasma display that canlower the firing voltage and the drive voltage (power consumption).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a plane discharge,alternating current type plasma display to which the sol solutionaccording to the present invention has been applied;

FIG. 2 is a schematic cross-sectional view of an opposed discharge,alternating current type plasma display to which the sol solutionaccording to the present invention has been applied;

FIG. 3 is a scanning electron photomicrograph (magnification: 50000times) showing the surface structure, in terms of particles, of aprotective layer constituted by a magnesium oxide film prepared inExample A1;

FIG. 4 is a scanning electron photomicrograph (magnification: 50000times) showing the surface structure, in terms of particles, of aprotective layer constituted by a magnesium oxide film formed by vacuumdeposition in Comparative Examples A4 and B3; and

FIG. 5 is a scanning electron photomicrograph (magnification: 50000times) showing the surface structure, in terms of fine particles, of aprotective layer constituted by a magnesium oxide film prepared inExample B1.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be described in more detail with reference tothe following preferred embodiments.

First Invention

The sol solution according to the first invention comprises a dispersionof fine particles of magnesium hydroxide bonded to a polyhydric alcoholor a derivative thereof (fine particles of a double hydroxide ofmagnesium) in an organic compound having at least one hydroxyl group oran organic solvent containing said organic compound. The fine particlesof the double hydroxide of magnesium may be produced by hydrolyzing amagnesium compound, convertible to magnesium hydroxide in the presenceof water, in water and a polyhydric alcohol or a derivative thereof inthe presence of a suitable catalyst to produce magnesium hydroxide andseparating the particles from the reaction medium. The particles of thedouble hydroxide of magnesium thus obtained is considered to have such astructure that magnesium hydroxide and the polyhydric alcohol orderivative thereof are bonded to each other through a covalent bond,ionic bond, coordinate bond, or a hydrogen bond. That is, it isconsidered that a composite of magnesium hydroxide and the polyhydricalcohol or derivative thereof having an indefinite ratio is formed.

An embodiment of preparation of fine particles of double hydroxide ofmagnesium is as follows.

TABLE A1 Example of preparartion of fine particles of double hydroxideof magnesium Pure water  50 parts by weight Ethylene glycol 150 parts byweight Magnesium acetate (tetrahydrate)  21 parts by weight Aqueousammonia (28 vol %)  6 parts by weight

Stirring of a mixed solution containing a composition as specified inTable A1 for one hr. at room temperature results in the formation of adouble hydroxide of magnesium which is then separated from the medium bya suitable method to give fine particles of a double hydroxide ofmagnesium.

This is merely one embodiment, and, in general, any magnesium compoundis usable so far as it can form magnesium hydroxide in the presence ofwater. The amount of the polyhydric alcohol or derivative thereof usedis about 50 to 950 parts by weight based on 100 parts by weight of themagnesium compound, and the amount of water used is about 25 to 1500parts by weight based on 100 parts by weight of the magnesium compound.The magnesium compound is hydrolyzed in the presence of a suitablecatalyst. The catalyst may be any one so far as it can accelerate thehydrolysis of the magnesium compound. For example, when the magnesiumcompound is a magnesium salt, a basic compound is used, as the catalyst,in an amount of not less than one equivalent, preferably about 1 to 5equivalents, based on one equivalent of the magnesium compound. When thecatalyst is in the form of an aqueous solution, such as aqueous ammonia,the water contained in the aqueous ammonia solution can be used as waterspecified in the above table.

Magnesium compounds usable instead of the magnesium compound given inthe table include, for example, magnesium salts of strong acids typifiedby magnesium chloride, magnesium nitrate, and magnesium sulfate,magnesium salts of weak acids typified by magnesium phosphate, magnesiumhydrogenphosphate, magnesium dihydrogenphosphate, magnesium carbonate,magnesium citrate, magnesium hydrogencitrate, and magnesium formate, andmagnesium salts of aliphatic carboxylic acids typified by magnesiumstearate and magnesium myristate.

In Table A1, ethylene glycol, which forms fine particles of a doublehydroxide of magnesium and serves also as a solvent, is one preferredexample of the polyhydric alcohol or derivative thereof, and, forexample, diethylene glycol, dihydric alcohols typified by2-methoxyethanol, 2-ethoxyethanol, and triethylene glycol andderivatives thereof, trihydric or higher polyhydric alcohols typified byglycerin, mixture systems thereof, and organic solvents containing theabove compounds, are usable instead of ethylene glycol.

Water as the solvent is an indispensable substance for the formation offine particles of the double hydroxide of magnesium. However, it isneedless to say that, in some cases, water need not be intentionallyadded on the premise that water contained in the aqueous ammonia isutilized.

Ammonia in the aqueous ammonia functions as a catalyst which acceleratesthe formation of magnesium hydroxide. Various ammonium salts typified byammonium acetate, ammonium amidosulfate, ammonium carbonate, ammoniumhydrogencarbonate, ammonium borate, diammonium citrate, ammoniumdihydrogenphosphate, diammonium hydrogenphosphate, triammoniumphosphate, ammonium formate, and ammonium tartrate, and amines typifiedby hydroxylamine, ethanolamine, and methanolamine are usable instead ofthis ammonia. The aqueous ammonia is particularly preferred from theviewpoint of attaining the object of the present invention because it isless likely to cause inclusion of impurities in the sol solution.

The double hydroxide of magnesium can be separated from the mixedsolution containing the double hydroxide of magnesium by any method, andexamples of methods usable herein include filtration, decantation, andcentrifugation. A cooling/centrifuge (Model 7930, manufactured by KubotaProduct Company Limited) was used in the separation of fine particles ofthe double hydroxide of magnesium produced from the compositionspecified in Table A1.

The fine particles of the double hydroxide of magnesium thus obtained isdispersed in an organic compound having at least one hydroxyl group oran organic solvent containing the above organic compound to prepare thesol solution of the present invention. The amount of the organic solventused for the double hydroxide of magnesium may be suitably set. Since,however, the coating thickness is regulated by the set value of thisparameter, this parameter should be carefully determined. For example,when the coatability of the resultant sol solution is taken intoconsideration, the organic solvent as the dispersing medium ispreferably 100 to 500 parts by weight based on 100 parts by weight ofthe double hydroxide of magnesium. When the solid content is excessivelylow, it is difficult to form a dense, continuous protective layer, whilewhen the solid content is excessively high, agglomeration precipitationof the fine particles of the double hydroxide of magnesium and adeterioration in uniformity of the protective layer are likely to occur.An embodiment of the sol solution is given in the following Table A2.

TABLE A2 Example of preparation of sol solution Double hydroxide of  5parts by weight magnesium Ethanol 10 parts by weight

The double hydroxide can be dispersed by conventional dispersing means,such as simple stirring, forced stirring, ball mill, sand mill, andultrasonic dispersion, and a homogeneous sol solution can be easilyprepared by the above dispersing means. An ultrasonic device (MODELUS-300T, manufactured by NIHON SEIKi CO) was used in the dispersion ofthe composition specified in Table A2.

Ethanol is one example of the dispersing medium in the sol solution, andany compound may be used as the dispersing medium so far as it has atleast one hydroxyl group. Dispersing media usable instead of ethanolinclude, for example, monohydric alcohols typified by methanol, n-propylalcohol, i-propyl alcohol, 1-butanol, and 2-butanol, dihydric alcoholsand derivatives thereof typified by ethylene glycol, diethylene glycol,2-methoxyethanol, 2-ethoxyethanol, and triethylene glycol, trihydric orhigher polyhydric alcohols typified by glycerin, and mixed solvents ofthese compounds or organic solvents containing at least one of the aboveorganic solvents having at least one hydroxyl group.

In the present invention, the reason why the fine particles of thedouble hydroxide of magnesium can be easily dispersed in the aboveorganic solvent by simple dispersing means is considered as follows.

Specifically, since the preparation of the fine particles of the doublehydroxide of magnesium is carried out in a polyhydric alcohol or aderivative thereof, such as ethylene glycol, the polyhydric alcohol orderivative thereof is bonded or coordinated to each magnesium/aquocomplex produced by hydrolysis, for example, by hydrogen bond to form akind of chelate complex. As the hydrolysis proceeds through the actionof the catalyst to increase the complex concentration, the chelatecomplexes associate with one another to form fine particles of thedouble hydroxide of magnesium. Since the formation of the fine particles(association of complexes) is carried out from a chelate complex of apolyhydric alcohol or a derivative thereof rather than from themagnesium/aquo complex, agglomeration of fine particles (agglomerationof complexes), which is likely to occur accompanying an increase incomplex concentration, is reduced.

The fine particles, which are associated chelate complexes, whendispersed in a hydroxyl-containing organic solvent, cause thehydroxyl-containing organic solvent to adsorb to the fine particlesthrough the hydroxyl group, so that a kind of protective film formed ofthe hydroxyl-containing organic solvent is formed on the surface ofindividual fine particles of the double hydroxide of magnesium. Thepresence of this protective film results in improved affinity of thefine particles for the organic solvent as a liquid medium for the solsolution and, at the same time, inhibits the agglomeration of fineparticles in the organic solvent, permitting the fine particles to beeasily and stably dispersed in the organic solvent. When this effectalone is taken into consideration, all organic compounds having ahydroxyl group are applicable. However, in order that, in the step ofdrying and firing after coating of the sol solution, the organic solventis removed to prepare a film of pure magnesium oxide, it is preferred touse a polyhydric alcohol having a relatively low boiling point or aderivative thereof and to use an organic solvent having a relatively lowboiling point as the solvent for the sol solution.

The surface of a magnesium oxide film formed from a sol solutionprepared without use of the polyhydric alcohol or derivative thereof inthe preparation of magnesium hydroxide has irregularities having a sizeof about 200 nm, that is, large roughness, whereas, as shown in FIG. 3,the surface of a magnesium oxide film formed from a sol solutionprepared using a polyhydric alcohol or a derivative thereof in thepreparation of magnesium hydroxide has very small irregularities and isrelatively even.

The reason why this phenomenon occurs is considered as follows.Specifically, for example, ethylene glycol is a solvent having a boilingpoint of 196° C., and, hence, an agglomerate of a magnesiumhydroxide/ethylene glycol complex after centrifugation is viscous andcreamy. On the other hand, magnesium hydroxide formed without using anypolyhydric alcohol or derivative thereof is fundamentally an agglomerateof magnesium hydroxide alone, and, hence, the agglomerate aftercentrifugation is not very viscous. That is, in the case of the solsolution with ethylene glycol added thereto, it is considered that, uponcoating of the sol solution, the effect of leveling by the agglomerateof fine particles is developed, rendering the surface of magnesium oxideafter firing even as shown in FIG. 3.

Further, the fine particles of a double hydroxide of magnesium in thesol solution according to the present invention are a complex and,hence, less likely to associate with one another and less likely toagglomerate, as compared with fine particles of magnesium hydroxideprepared without using any polyhydric alcohol or derivative thereof.Therefore, the sol solution is stable even when the concentration of thesol solution is increased, which is advantageous in storage stability,coatability, and the steps of drying and firing.

Regarding the solvent for the sol solution, the use of water or asolvent not having any hydroxyl group, instead of the above alcoholicorganic solvent, such as toluene or hexane, makes it difficult to stablydisperse the fine particles of the double hydroxide of magnesium in thesolvent, causing agglomeration and precipitation. For this reason, theorganic solvent having a hydroxyl group, or an organic solventcontaining at least one organic solvent of the above type is preferablyused as the solvent for the sol solution.

Properties of sol solutions used in the present invention andComparative Examples A1 and A2 described below are summarized, asexperimental results supporting the above observation, in Table A3.

TABLE A3 Properties of various sol solutions Average particle SolidStability Sample size content with time Ex. A1 836 nm 18.5 wt % GoodComp. Ex. A1 Immeasurable  2.0 wt % Poor Comp. Ex. A2 Immeasurable 17.9wt % Poor

The sol solution of Comparative Example A1 is one prepared by dispersingfine particles of magnesium hydroxide, prepared without adding ethyleneglycol, in pure water instead of ethanol. The sol solution ofComparative Example A2 is one prepared by dispersing fine particles of adouble hydroxide of magnesium in pure water instead of ethanol at thetime of preparation of the sol solution.

Regarding the evaluation of the properties, the average particle size ofthe fine particles in the sol solution was measured and evaluated with alaser particle analyzer (PAR-III, manufactured by Otsuka Denshi K.K.)under conditions of pinhole φ 0.2. The solid content was determined byplacing a given weight of each sol solution in a sample tube, drying thesample at 120° C. or 3 hr. and measuring the weight proportion of theresidue. Regarding the stability with the time, upon the preparation ofthe sol solution, the sol solution was allowed to stand for one day toevaluate whether or not a precipitate occurred.

A method for film formation using the sol solution according to thepresent invention will be described with reference to the production ofan alternating current type plasma display shown in the drawing by wayof example.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing the structure of a planedischarge, alternating current type plasma display according to apreferred embodiment of the present invention.

In FIG. 1, numerals 1 and 2 respectively designate a front substrate anda back substrate disposed parallel and opposite to each other with a gasdischarge space 3 sandwiched therebetween. These front substrate 1 andback substrate 2 each are constituted by a glass sheet having apredetermined thickness.

A pair of electrodes, an electrode X 4 a and an electrode Y 4 b, areformed on the front substrate 1 in its surface facing the back substrate2. These pair of electrodes are covered with a dielectric layer 5 madeof glass, and the dielectric layer 5 is covered with a protective layer6 constituted by a magnesium oxide film formed by coating the solsolution of the present invention and drying and firing the coating.

An address electrode 7, a barrier 8, and a phosphor layer 9 are formedon the back substrate 2 in its side facing the front substrate 1.Further, if necessary, for example, a titanium dioxide film (a highrefractive index layer) 10 and a silicon dioxide film (a low refractiveindex layer) 11 are formed as a reflection preventive layer on the frontsubstrate 1.

FIG. 2 is a schematic diagram showing the structure of an opposeddischarge, alternating current type plasma display. In FIG. 2, anelectrode X 4 a is formed on the surface of a front substrate 1 in itssurface facing a back substrate 2, the electrode X 4 a is covered with adielectric layer 5 made of glass, and the dielectric layer 5 is coveredwith a protective layer 6 formed of magnesium oxide prepared from a solsolution described below.

An electrode Y 4 b, a dielectric layer 5, a protective layer 6 formed inthe same manner as described above, and a barrier 8 and a phosphor 9 areprovided on the back substrate 2 in its surface facing the frontsubstrate 1.

Further, if necessary, for example, a titanium dioxide film (a highrefractive index layer) 10 and a silicon dioxide film (a low refractiveindex layer) 11 are formed as a reflection preventive layer on the frontsubstrate 1.

A process for producing an alternating current type plasma displayaccording to the present invention is characterized by coating the abovesol solution on the above dielectric layer and drying and firing thecoating to form the above protective layer.

The sol solution may be coated on the dielectric layer by any coatingmethod, and various coating methods, for example, spin coating, dipcoating, spray coating, roll coating, meniscus coating, bar coating,curtain flow coating, bead coating, and casting, are applicable.

Drying and firing of a wet coating formed by the coating method resultin the formation of a transparent magnesium oxide film as the protectivelayer, and the magnesium oxide film has high adhesion to the dielectriclayer 5. The drying may be performed under conditions of such atemperature and a period of time that the organic solvent component inthe wet coating is substantially removed by evaporation. For example,drying at a temperature of about 200 to 300° C. for about 1 to 3 hr.suffices for satisfactory results. Preferably, the firing can beperformed at a temperature of about 350 to 550° C. for about 1 to 5 hr.Excessively severe firing conditions pose problems attributable tosoftening of the dielectric layer, such as delamination or cracking ofthe protective layer, On the other hand, when the firing isunsatisfactory, no protective layer having desired properties can beformed. Preferably, the drying and the firing are continuouslyperformed. However, they may be separately performed. In a workingexample described below, the drying and the firing were continuouslyperformed, that is, at 300° C. for one hr. and 400° C. for one hr.

In the present invention, a scanning type electron photomicrograph(magnification: 50000 times, device used: S-800, manufactured by JapanElectric Optical Laboratory) showing the surface structure, in terms ofparticles, of the protective layer constituted by a magnesium oxide filmformed as described above is shown in FIG. 3. For comparison, aphotomicrograph of the surface of a protective layer prepared by vacuumdeposition in Comparative Example A4 described below is shown in FIG. 4.The measurement was conducted under conditions of accelerated voltage 5kV, working distance 5 mm, beam monitor aperture No. 2, and objectivemovable aperture No. 3. As is apparent from FIG. 3, the protective layerconstituted by the magnesium oxide film according to the presentinvention is such a dense, continuous film that the surface of theprotective layer is constituted by particles, in a particulate form,having an average particle diameter of 30 nm. The form of the surface ofthe above film is clearly different from that of a magnesium oxide film,formed by vacuum deposition, constituted by flaky particles having anaverage particle diameter of 200 nm (FIG. 4).

In the present invention, the surface area of fine particles ofmagnesium oxide constituting the protective layer should be increasedfrom the viewpoint of increasing the secondary electron emission ratioin the alternating current type plasma display, and, for this purpose,desirably, the diameter of fine particles of magnesium oxide is in therange of from 5 to 100 nm, preferably in the range of from 5 to 30 nm.The particle diameter is reduced to not more than 100 nm to reduce thegap between the particles and to enable an magnesium oxide film to beefficiently formed even by a conventional heat treatment process.Although the thickness of the magnesium oxide film is not particularlylimited, it is preferably not more than 10 μm from the viewpoint oftransparency, particularly preferably not more than 1 μm.

In the protective layer in the alternating current type plasma displaythus obtained, a thin film having a thickness of not more than 1 μm canbe realized, and such a film thickness cannot be realized by a pasteusing the conventional binder.

The sol solution according to the present invention can be used as aprotective layer provided on the dielectric layer as a substrate in analternating current type plasma display. Further, a magnesium oxide filmcan be formed on other substrates according to purposes.

Second Invention

The sol solution according to the second invention comprises adispersion of an agglomerate of fine particles of magnesium hydroxide inan organic solvent containing at least one hydroxyl-containing organiccompound. The agglomerate of fine particles of magnesium hydroxide maybe produced by hydrolyzing a magnesium compound, convertible tomagnesium hydroxide in the presence of water, in the presence of asuitable catalyst to produce magnesium hydroxide and separating theagglomerate from the reaction medium.

An embodiment of preparation of the agglomerate of fine particles ofmagnesium hydroxide is as follows.

TABLE B1 Example of preparation of fine particles of magnesium hydroxidePure water 200 parts by weight Magnesium acetate (tetrahydrate)  21parts by weight Aqueous ammonia (28 vol %)  6 parts by weight

Stirring of a mixed solution containing a composition as specified inTable B1 for one hr. at room temperature results in the formation of aprecipitate of an agglomerate of fine particles of magnesium hydroxidewhich is then separated from water as the reaction medium by a suitablemethod to give an agglomerate of fine particles of magnesium hydroxide.

This is merely one embodiment, and, in general, any magnesium compoundis usable so far as it can form magnesium hydroxide in the presence ofwater. The catalyst may be any one so far as it can accelerate thehydrolysis of the magnesium compound. For example, when the magnesiumcompound is a magnesium salt, a basic compound is used, as the catalyst,in an amount of not less than one equivalent, preferably about 1 to 5equivalents, based on one equivalent of the magnesium compound. When thecatalyst is in the form of an aqueous solution, such as aqueous ammonia,the water contained in the aqueous ammonia solution can be used as waterspecified in the above table.

Magnesium compounds usable herein include those exemplified above inconnection with the first invention.

Ammonia in the aqueous ammonia functions as a catalyst which acceleratesthe formation of magnesium hydroxide. Various ammonium salts typified byammonium acetate, ammonium amidosulfate, ammonium carbonate, ammoniumhydrogencarbonate, ammonium borate, diammonium citrate, ammoniumdihydrogenphosphate, diammonium hydrogenphosphate, triammoniumphosphate, ammonium formate, and ammonium tartrate, and amines typifiedby hydroxylamine, ethanolamine, and methanolamine are usable instead ofthis ammonia. The aqueous ammonia is particularly preferred from theviewpoint of attaining the object of the present invention because it isless likely to cause inclusion of impurities in the sol solution.

The agglomerate of fine particles of magnesium hydroxide can beseparated from the mixed solution containing the precipitate of anagglomerate of fine particle of magnesium hydroxide by any method, andexamples of methods usable herein include filtration, decantation, andcentrifugation. A cooling/centrifuge (Model 7930, manufactured by KubotaProduct Company Limited) was used to in the separation of theagglomerate of fine particles of magnesium hydroxide produced from thecomposition specified in Table B1.

The fine particles of the agglomerate of magnesium hydroxide thusobtained is dispersed in an organic solvent containing at least onehydroxyl-containing organic compound to prepare the sol solution of thepresent invention. The amount of the organic solvent used for dispersingthe agglomerate of fine particle of magnesium hydroxide may be suitablyset. Since, however, the coating thickness is regulated by the set valueof this parameter, this parameter should be carefully determined. Forexample, when the coatability of the resultant sol solution is takeninto consideration, the organic solvent as the dispersing medium ispreferably 100 to 1500 parts by weight based on 100 parts by weight ofthe fine particles of magnesium hydroxide. When the solid content isexcessively low, it is difficult to form a dense, continuous protectivelayer, while when the solid content is excessively high, agglomerationprecipitation of the fine particles and a deterioration in uniformity ofthe protective layer are likely to occur. An embodiment of the solsolution is given in the following Table B2.

TABLE B2 Example of preparation of sol solution Agglomerate of fineparticles  1 part by weight of magnesium hydroxide Ethanol 10 parts byweight

The agglomerate can be dispersed by conventional dispersing means, suchas simple stirring, forced stirring, ball mill, sand mill, andultrasonic dispersion, and a homogeneous sol solution can be easilyprepared by the above dispersing means. An ultrasonic device (MODELUS-300T, manufactured by NIHON SEIKi CO) was used in the dispersion ofthe composition specified in Table B2.

Ethanol is one example of the dispersing medium in the sol solution, andany organic compound having at least one hydroxyl group or any organicsolvent containing at least one compound of the above type may be usedas the dispersing medium. Dispersing media usable instead of ethanolinclude, for example, monohydric alcohols typified by methanol, n-propylalcohol, i-propyl alcohol, 1-butanol, and 2-butanol, dihydric alcoholsand derivatives thereof typified by ethylene glycol, diethylene glycol,2-methoxyethanol, 2-ethoxyethanol, and triethylene glycol, trihydric orhigher polyhydric alcohols typified by glycerin, aromatic compounds,such as phenol and cresol, and mixed solvents of these compounds ororganic solvents containing at least one of the above organic solventshaving at least one hydroxyl group.

In the present invention, the reason why the agglomerate of fineparticles of magnesium hydroxide can be easily dispersed in the aboveorganic solvent by simple dispersing means is considered as follows.

Specifically, when an agglomerate of fine particles of magnesiumhydroxide is dispersed in a hydroxyl-containing organic solvent, thehydroxyl-containing organic solvent is bonded, coordinated, or adsorbedto an agglomerate (secondary agglomerate) of individual fine particlesof magnesium hydroxide by taking advantage of the hydroxyl group througha hydrogen bond or the like to form fine particles covered with a kindof a protective film of the hydroxyl-containing organic solvent formedon the surface of the particles. It is considered that the presence ofthis protective film results in improved affinity of the fine particlesfor the organic solvent as a dispersing medium for the sol solution and,at the same time, inhibits the agglomeration of fine particles per se inthe organic solvent, permitting the fine particles to be easily andstably dispersed in the organic solvent. When this effect alone is takeninto consideration, all organic compounds having a hydroxyl group areapplicable. However, in order that, in the step of drying and firingafter coating of the sol solution, the organic solvent is removed toprepare a film of pure magnesium oxide, it is preferred to use ahydroxyl-containing organic solvent having a relatively low boilingpoint as the solvent for the sol solution.

Regarding the solvent for the sol solution, the use of water or asolvent not having any hydroxyl group, such as toluene or hexane,instead of the above alcoholic organic solvent, makes it difficult tostably disperse the fine particles of magnesium hydroxide in thesolvent, causing agglomeration and precipitation. For this reason, theorganic solvent having a hydroxyl group, or an organic solventcontaining at least one organic solvent of the above type is preferablyused as the solvent for the sol solution.

Properties of sol solutions used in the present invention andComparative Example B1 described below are summarized, as experimentalresults supporting the above observation, in Table B3.

TABLE B3 Properties of various sol solutions Average particle SolidStability Sample size content with time Ex. B1 873 nm 0.9 wt % GoodComp. EX. B1 Immeasurable 2.0 wt % Poor

The sol solution of Comparative Example B1 is one prepared by dispersingan agglomerate of fine particles of magnesium hydroxide specified inTable B1, prepared without adding ethylene glycol, in pure water insteadof ethanol.

Properties of the sol solutions were evaluated in the same manner as inthe first invention.

A film can be formed using the sol solution according to the second,invention in the same manner as described above in connection with anembodiment of the production of alternating current type plasmadisplays, according to the present invention, shown in FIGS. 1 and 2.

In the present invention, a scanning type electron photomicrograph(magnification: 50000 times, device used: S-800, manufactured by JapanElectric Optical Laboratory) showing the surface structure, in terms ofparticles, of the protective layer constituted by a magnesium oxide filmformed as described above is shown in FIG. 5. For comparison, aphotomicrograph of the surface of a protective layer prepared by vacuumdeposition in Comparative Example B3 described below is shown in FIG. 4.The measurement was conducted under conditions of accelerated voltage 5kV, working distance 5 mm, beam monitor aperture No. 2, and objectivemovable aperture No. 3. As is apparent from FIG. 5, the protective layerconstituted by the magnesium oxide film according to the presentinvention is such a dense, continuous film that the surface of theprotective layer is constituted by particles, in a particulate form,having an average particle diameter of 30 nm. The form of the surface ofthe above film is clearly different from that of a magnesium oxide film,formed by vacuum deposition, constituted by flaky particles having anaverage particle diameter of 200 nm (FIG. 4).

In the present invention, the surface area of fine particles ofmagnesium oxide constituting the protective layer should be increasedfrom the viewpoint of increasing the secondary electron emission ratioin the alternating current type plasma display, and, for this purpose,desirably, the diameter of fine particles of magnesium oxide is in therange of from 5 to 100 nm, preferably in the range of from 5 to 30 nm.The particle diameter is reduced to not more than 100 nm to reduce thegap between the particles, thereby increasing the surface area, and toenable an magnesium oxide film to be efficiently formed even by aconventional heat treatment process. Although the thickness of themagnesium oxide film is not particularly limited, it is preferably notmore than 10 μm from the viewpoint of transparency, particularlypreferably not more than 1 μm.

In the protective layer in the alternating current type plasma displaythus obtained, a thin film having a thickness of not more than 1 μm canbe realized, and such a film thickness cannot be realized by a pasteusing the conventional binder.

The sol solution according to the present invention can be used as aprotective layer provided on the dielectric layer as a substrate in analternating current type plasma display. Further, a magnesium oxide filmcan be formed on other substrates according to purposes.

Third Invention

The use of an organometallic compound, such as a magnesium alkoxide, asa starting compound for the magnesium hydroxide sol can offer magnesiumoxide having relatively good adhesion to the substrate and uniformity inthe film. Magnesium alkoxides and the like, however, involve a problemof cost because they are relatively expensive. Further, since thereactivity of the magnesium alkoxides is relatively high, the reactionshould be carefully controlled. Furthermore, the life of the resultantmagnesium hydroxide is so short that careful handling is necessary.

For the above reason, in general, an organometallic salt, such asmagnesium acetate, or an inorganic metal salt, such as magnesiumchloride or magnesium sulfate, is used instead of the magnesium alkoxideand the like. However, the use of a large amount of an alkali, such asammonia, is necessary to prepare a magnesium hydroxide sol, requiringthe step of removal of the alkali and the like. This often renders theproduction process complicated. Bringing the particle diameter to notmore than 1 μm is preferred from the viewpoints of improving thetransparency of the magnesium oxide layer and the adhesion of magnesiumhydroxide to the substrate. For attaining this purpose, the addition ofan additive, such as ethylene glycol, to the magnesium hydroxide solproduction system is recommended from the viewpoint of inhibiting thegrowth of the particles. The addition of the above additive ofteninhibits the production of the magnesium hydroxide sol per se, resultingin lowered yield of the sol.

According to the third invention, when a sol solution of colloidalparticles of magnesium acetate is used, for example, in the formation ofa protective layer in an alternating current type PDP, a film can beformed on a panel having a large area without the need to use expensiveequipment such as required in the vacuum process. The film thus formedhas film strength, adhesion, transparent, protective properties andother film properties, and the of such a film enables the production ofan alternating current type PDP that can lower the firing voltage andthe drive voltage (power consumption).

The third invention is based on the present inventor's finding, that is,such finding that although a commercially available magnesium acetate isgenerally in tetrahydrate form and is completely soluble in water,partially dehydrated hydrous magnesium acetate and anhydrous magnesiumacetate produced by partially or completely removing the water ofhydration is insoluble in water and that the addition of the abovemagnesium acetate to water causes a part of the surface of particles toundergo hydrolysis to form hydrophilic magnesium hydroxide which isdispersed as stable colloidal particles in water.

The starting compound for forming the magnesium acetate sol according tothe present invention is magnesium acetate tetrahydrate, and the removalof 20% by weight of the water of hydration is preferred from theviewpoint of forming stable colloidal particles in a medium composedmainly of water. Particularly preferably, the magnesium acetate iscompletely dehydrated to form anhydrous magnesium acetate. Thedehydration may be performed by any method without limitation. Forexample, heating under reduced pressure is simple and, hence, ispreferred.

The medium for dispersing the anhydrous or partially dehydratedmagnesium acetate is composed mainly of water, that is, 100% water or amixed solution of water and a water-soluble solvent. In particular, theuse of alcohols, glycols, esters, ethers, or organic solvents having ahigh dielectric constant, such as propylene carbonate andγ-butyrolactone, are preferred from the viewpoint of allowing thehydrolysis of at least part of magnesium acetate on the surface of theparticles to homogeneously and efficiently proceed, thereby forming astable sol solution. The amount of the above organic solvent added towater is not particularly limited so far as the proportion of water inthe mixed solution is not 50% by weight or less. The content (solidbasis) of the anhydrous or partially hydrated magnesium acetate in themedium composed mainly of water is usually 0.1 to 90% by weight.However, it is preferably in the range of from 0.1 to 60% by weight fromthe viewpoint of forming a homogeneous sol solution.

A dispersion stabilizer may be incorporated into the sol solution inorder to enhance the stability of the sol solution. Examples ofpreferred dispersion stabilizers include water-soluble polymers, forexample, celluloses, such as hydroxypropyl methylcellulose andhydroxypropylmethyl cellulose, acrylic polymers, such as polyacrylicacid and polyacrylamide, and vinyl polymers, such as polyvinyl alcoholand a partially saponified product thereof. They may be used alone or amixture of two or more, and the content of the dispersion stabilizer inthe sol solution is usually in the range of from 0.01 to 10% by weight.

The sol solution containing magnesium acetate particles dispersed in acolloidal form can be prepared by adding anhydrous or partially hydratedmagnesium acetate to medium composed mainly of water with awater-soluble solvent and a dispersion stabilizer being optionallydissolved therein and stirring the mixture. A reaction for forming thecolloid smoothly proceeds at room temperature. However, heating to, forexample, 30 to 80° C. can accelerate the reaction.

Further, in order to regulate the diameter of colloidal particles in apredetermined range, ethylene glycol or a derivative of ethylene glycol,such as diethylene glycol or methyl CELLOSOLVE® (2-ethoxyethanol), maybe added to the above reaction system. The amount of ethylene glycol ora derivative thereof added is usually in the range of from 0.01 to 80%by weight. As the amount of ethylene glycol or a derivative thereofadded increases, the diameter of colloidal particles decreases. Theparticle diameter distribution can be broadened or narrowed to giveparticles having uniform diameter, according to the purpose of use(application) of the sol solution of the present invention.

Therefore, for example, a sol solution containing a mixture of largecolloidal particles with small colloidal particles may be used to form adense magnesium oxide layer (film), or alternatively a sol solutioncontaining colloidal particles having uniform diameter may be used toform a thin layer of magnesium oxide.

The reason why the diameter of the sol particles can be regulated isbelieved to reside in that an oxygen atom within the molecule ofethylene glycol or a derivative thereof is coordinated to the magnesiumatom of magnesium acetate to stabilize the dispersibility of colloidalparticles of magnesium acetate. In the sol solution of the presentinvention, the diameter of the colloidal particles is in the range offrom 3 to 300 nm.

In the magnesium acetate sol solution according to the presentinvention, at least a part of magnesium acetate on the surface of thecolloidal particles undergoes hydrolysis to form a magnesium hydroxide,rendering the sol solution stable. Further, when the magnesium acetatesol solution of the present invention is coated on a substrate to form afilm, as compared with a film formed using a conventional coating liquidwith magnesium hydroxide forcibly formed by adding aqueous ammonia to amagnesium acetate solution, the adhesion between the colloidal particlelayer (film) and the substrate is significantly improved realizing adense magnesium oxide layer having uniform particle diameter or thelike.

The method for film formation according to the present inventioncomprises the steps of: coating the above magnesium acetate sol solutionon a substrate; drying the resultant magnesium acetate film; andheat-treating the dried magnesium acetate film to form a magnesium oxidefilm.

The sol solution may be coated on the substrate by any coating method,and various coating methods, for example, spin coating, dip coating,spray coating, roll coating, meniscus coating, and bar coating, areapplicable.

Drying and heat treatment (firing) of a wet coating formed by thecoating method result in the formation of a transparent magnesium oxidefilm adhered to the substrate. The drying may be performed underconditions of such a temperature and a period of time that the water andthe water-soluble solvent in the wet coating is substantially removed byevaporation. For example, drying at a temperature of about 100 to 200°C. for about 1 to 2 hr. suffices for satisfactory results. Preferably,the firing can be performed at a temperature of about 300 to 600° C. forabout 1 to 5 hr. The heat treatment at the above temperature ispreferred from the viewpoint of completely converting the magnesiumhydroxide to magnesium oxide by firing and enhancing the crystallinityof the film to improve the film properties.

In the present invention, the surface area of fine particles ofmagnesium oxide constituting the protective layer should be increasedfrom the viewpoint of increasing the secondary electron emission ratioin the alternating current type PDP, and, for this purpose, desirably,the diameter of fine particles of magnesium oxide is in the range offrom 5 to 100 nm, preferably in the range of from 5 to 30 nm. Theparticle diameter is reduced to not more than 100 nm to reduce the gapbetween the particles, thereby increasing the surface area, and toenable an magnesium oxide film to be efficiently formed even by aconventional heat treatment process. Although the thickness of themagnesium oxide film is not particularly limited, it is preferably notmore than 10 μm from the viewpoint of transparency, particularlypreferably not more than 1 μm.

According to the magnesium oxide film prepared by the method of thepresent invention, a thin film, having a thickness of not more than 1μm, of magnesium oxide alone can be produced without the use of anybinder. The production of the above thin film has been unattainable byusing the conventional magnesium oxide printing paste.

The formation of a protective layer having a thickness of not more than2 μm in the case of, for example, an alternating current PDP is regardedas the most important necessary and sufficient condition from thepractical viewpoint. The film formed by the method for film formationaccording to the present invention is formed in a thickness of not morethan 1 μm and, hence, is particularly suitable as a protective layer(film) for a dielectric layer in the alternating current type PDP. Theapplication of the film is not limited to this only.

EXAMPLE A1

An opposed discharge alternating current type plasma display shown inFIG. 2 was prepared as follows. A 0.2 μm-thick electrode X 4 a as achromium electrode was vacuum-deposited on a front substrate 1 made ofglass. A 0.8 μm-thick dielectric layer 5 was then vacuum-deposited, anda sol solution specified in Table 2 was used to form a 0.2 μm-thickprotective layer 6 of a magnesium oxide film on the dielectric layer 5.Thereafter, a 150 μm-thick barrier 8 was formed on the protective layer6 by screen printing, and a fluorescent material was coated on thebarrier 8 to form a 10 μm-thick phosphor layer 9.

Separately, a chromium electrode was vacuum-deposited on a backsubstrate 2 made of glass and patterned to form an electrode Y 4 b, anda dielectric layer 5 was vacuum-deposited on the electrode Y 4 b.Thereafter, a 0.2 μm-thick protective layer 6 of a magnesium oxide filmwas formed in the same manner as described above in connection with themagnesium oxide film. The film thickness of the chromium electrode(electrode Y 4 b) on the back substrate 2 was 0.2 μm, and the filmthickness of the dielectric layer 5 was 0.8 μm.

Both the substrates thus obtained were laminated to each other so thatthe protective layers faced each other. 500 Torr of He—Xe (1.1%) Penninggas was sealed into a space defined by the barrier 8 to prepare anopposed discharge alternating current type plasma display of the presentinvention.

COMPARATIVE EXAMPLE A1

An attempt to prepare an opposed discharge alternating current typeplasma display was made in the same manner as in the above example,except that, in the preparation of fine particles of magnesium oxide, asol solution prepared by dispersing fine particles of magnesiumhydroxide, prepared without the addition of ethylene glycol, in purewater instead of ethanol, was used instead of the sol solution in theabove example.

COMPARATIVE EXAMPLE A2

An attempt to prepare an opposed discharge alternating current typeplasma display was made in the same manner as in the above example,except that, in the preparation of the sol solution, fine particles of adouble hydroxide of magnesium were dispersed in pure water instead ofethanol.

COMPARATIVE EXAMPLE A3

An opposed discharge alternating current type plasma display wasprepared in the same manner as in the above example, except that a pasteof magnesium oxide was coated by screen printing to form the protectivelayer 6 of a magnesium oxide film. The paste of magnesium oxide wasprepared by dispersing a magnesium oxide powder having an averageprimary particle diameter of 0.18 to 0.25 μm (UBE 2000A, manufactured byUbe Industries, Ltd.) in a Si-based binder (10 parts by weight of SiO₂)to a concentration of 26% by weight and modifying the viscosity to 7,000cps/25° C.

Printing was performed under conditions of plate gap 2.3 mm, squeegeeindentation 1 mm, scraper indentation 1 μm, screen mesh 400, andemulsion thickness 10 μm. Sintering was performed under atmosphericpressure at a temperature of 580° C. for one hr.

COMPARATIVE EXAMPLE A4

An opposed discharge alternating current type plasma display wasprepared in the same manner as in the above example, except that theprotective layer 6 of a magnesium oxide film was formed by vacuumdeposition. In the vacuum deposition, an electron beam depositingmachine (EX-900-C16, manufactured by Ulvac Japan Limited) was used toform a film under conditions of substrate temperature 300° C. and filmformation rate 0.5 nm/sec to a thickness of 0.5 μm.

Before paneling, the alternating current type plasma displays(hereinafter referred to as “panels”) prepared in the example and thecomparative examples were evaluated for the properties of the magnesiumoxide film constituting the protective layer. The results are summarizedin Table A4. The adhesion and the film strength given in the table wereevaluated by a scratch tester and a pencil hardness test. Thetransparency and the crack were visually inspected. The film thicknesswas measured with a profilometer (αStep 300, manufactured by TencorInstruments Japan Co., Ltd.).

TABLE A4 Properties of various magnesium oxide films Form of Averagefine particle Film Film Sample particles diameter Adhesion strengthTransparency Crack thickness Ex. A1 Particulate  30 nm Good Good GoodFree 0.2 μm Comp. Ex. A1 — — Failure Failure Failure — — Comp. Ex. A2 —— Failure Failure Failure — — Comp. Ex. A3 Cubic 800 nm Good GoodFailure Free 3.0 μm Comp. Ex. A4 Flaky 200 nm Good Good Good Free 0.5 μm

In Comparative Examples A1 and A2, since the wetting property of eachsol solution was poor, conditions such as coating speed were varied.However, a wet coating could not be evenly formed on the dielectriclayer 5. This led to the evaluation results given in Table A4. Forcomparative Example A3, the film thickness was 3 μm, i.e., larger thanthat in the other comparative examples and the example, and the film waswhitish and had poor transparency. Comparative Example A4 had nosignificant problem.

Then, for the opposed discharge alternating current type plasma displaysexcept for those in Comparative Examples A1 and A2 wherein no protectivelayer could be formed, an alternating current pulse having a waveform ofa drive frequency 15 kHz and a duty ratio of 23% was used to measure thefiring voltage Vf and the minimum maintaining voltage Vsm.

After setting at the firing voltage, continuous discharge was performedfor 60 min. Thereafter, the minimum maintaining voltage was measured andcompared with the initial minimum maintaining voltage to investigate achange in voltage with the elapse of time. Thus, the life property ofthe panels was evaluated. The results are summarized in Table A5.

TABLE A5 PDP properties ot various magnesium oxide films Minimum Firingmaintaining Sample voltage voltage Life Ex. A1 155 V 110 V Good Comp.Ex. A3 240 V 175 V Somewhat poor Comp. Ex. A4 160 V 105 V Good

As is apparent from Table A5, for the panel prepared in the example ofthe present invention, the firing voltage was 155 V, the minimummaintaining voltage was 110 V, and no variation in the minimummaintaining voltage was observed, that is, no problem was found in thelife property. For the panels prepared in the comparative examples, thepanel prepared in Comparative Example A3 had a firing voltage of 240 Vand a minimum maintaining voltage of 175 V which were higher than thosein the panel of the example of the present invention, necessitating ahigher panel drive voltage. Regarding the life property, 30 min. aftersetting at the firing voltage, an increase in the minimum maintainingvoltage was observed. For Comparative Example A4, the firing voltage was160 V, the minimum maintaining voltage was 105 V, and no variation inthe minimum maintaining voltage was observed, that is, no problem wasfound in the life property.

The results for the evaluation of the panel prepared in the example ofthe present invention, in comparison with the panels prepared in theother comparative examples, especially the panel prepared in ComparativeExample A4 by vacuum deposition which had achieved satisfactory resultsin the prior art, show that the panel developed functions inherent inthe magnesium oxide film and was satisfactory.

From the above results, it is apparent that the protective layer of amagnesium oxide film in the opposed discharge alternating current typeplasma display prepared in the example of the present invention exhibitssatisfactory properties in alternating current type plasma displays.

The use of the sol solution and the film forming method according to thepresent invention, for example, in the construction of an alternatingcurrent type plasma display, can provide a protective layer of acontinuous film of magnesium oxide constituted by granular particleshaving a particle diameter in the range of from 5 to 100 nm. Therefore,the protective layer has high transparency, and the thickness of theprotective layer can be reduced. Further, since the film strength ishigh, there is no fear of the dielectric layer being exposed bycracking. Therefore, a lowering in drive voltage can be accelerated,enabling a reduction in cost of drive circuit, which in turn leads to areduction in cost of the body of the plasma display.

Further, since the protective layer can be formed by coating a solsolution, the film can be formed in a larger area at a lower cost ascompared with film formation using a thin film process, realizing theproduction of a plasma display having a large area (for example, about40 in. in terms of diagonal distance) at a low cost.

Furthermore, in the sol solution according to the present invention,since an organic compound having at least one hydroxyl group or anorganic solvent containing at least one compound of the above type isused as the solvent, the sol solution is stable for a long period oftime without causing agglomeration of fine particles of a doublehydroxide of magnesium. Therefore, a magnesium oxide film prepared bycoating the sol solution and sintering the coating is constituted byrefined particles, improving the transparency of the resultantprotective layer.

Further, according to the film forming method of the present invention,since an organic solvent is used as the solvent for the sol solution,upon sintering, no organic matter is left as a residue in the magnesiumoxide film, realizing the preparation of a protective layer constitutingof pure magnesium oxide.

In general, when the thickness of the protective layer in an alternatingcurrent type plasma display is thick, for example, as thickness as about10 μm, the wall charge effect, a source of memory function, which is oneof the important properties of the alternating current type plasmadisplay, is deteriorated, necessitating a higher drive voltage.Consequently, specifications for transistors used in the drive circuitshould cope with the use of high voltage. For this reason, the formationof a protective layer having a thickness of not more than 2 μm is themost important necessary and sufficient condition from the practicalviewpoint. According to the present invention, the protective layerhaving a thickness of not more than 2 μm can be satisfactorily formed.

EXAMPLE B1

An opposed discharge alternating current type plasma display shown inFIG. 2 was prepared as follows. A 0.2 μm-thick electrode X 4 a as achromium electrode was vacuum-deposited on a front substrate 1 made ofglass. A 0.8 μm-thick dielectric layer 5 was then vacuum-deposited, anda sol solution specified in Table B2 was used to form a 0.2 μm-thickprotective layer 6 of a magnesium oxide film on the dielectric layer 5.Thereafter, a 150 μm-thick barrier 8 was formed on the protective layer6 by screen printing, and a fluorescent material was coated on thebarrier 8 to form a 10 μm-thick phosphor layer 9.

Separately, a chromium electrode was vacuum-deposited on a backsubstrate 2 made of glass and patterned to form an electrode Y 4 b, anda dielectric layer 5 was vacuum-deposited on the electrode Y 4 b.Thereafter, a 0.2 μm-thick protective layer 6 of a magnesium oxide filmwas formed in the same manner as described above in connection with themagnesium oxide film. The film thickness of the chromium electrode(electrode Y 4 b) on the back substrate 2 was 0.2 μm, and the filmthickness of the dielectric layer 5 was 0.8 μm.

Both the substrates thus obtained were laminated to each other so thatthe protective layers faced each other. 500 Torr of He—Xe (1.1%) Penninggas was sealed into a space defined by the barrier 8 to prepare anopposed discharge alternating current type plasma display of the presentinvention.

COMPARATIVE EXAMPLE B1

An attempt to prepare an opposed discharge alternating current typeplasma display was made in the same manner as in the above example B1,except that, in the preparation of the sol solution, agglomerates offine particles of magnesium hydroxide were dispersed in pure waterinstead of ethanol.

COMPARATIVE EXAMPLE B2

An opposed discharge alternating current type plasma display wasprepared in the same manner as in the above example B1, except that apaste of magnesium oxide was coated by screen printing to form theprotective layer 6 of a magnesium oxide film. The paste of magnesiumoxide was prepared by dispersing a magnesium oxide powder having anaverage primary particle diameter of 0.18 to 0.25 μm (UBE 2000A,manufactured by Ube Industries, Ltd.) in a Si-based binder (10 parts byweight of SiO₂) to a concentration of 26% by weight and modifying theviscosity to 7,000 cps/25° C.

Printing was performed under conditions of plate gap 2.3 mm, squeegeeindentation 1 mm, scraper indentation 1 μm, screen mesh 400, andemulsion thickness 10 μm. sintering was performed under atmosphericpressure at a temperature of 580° C. for one hr.

COMPARATIVE EXAMPLE B3

An opposed discharge alternating current type plasma display wasprepared in the same manner as in the above example B1, except that theprotective layer 6 of a magnesium oxide film was formed by vacuumdeposition. In the vacuum deposition, an electron beam depositingmachine (EX-900-C16, manufactured by Ulvac Japan Limited) was used toform a film under conditions of substrate temperature 300° C. and filmformation rate 0.5 nm/sec to a thickness of 0.5 μm.

Before paneling, the alternating current type plasma displays(hereinafter referred to as “panels”) prepared in the example and thecomparative examples were evaluated for the properties of the magnesiumoxide film constituting the protective layer. The results are summarizedin Table B4. The adhesion and the film strength given in the table wereevaluated by a scratch tester and a pencil hardness test. Thetransparency and the crack were visually inspected. The film thicknesswas measured with a profilometer (αStep 300, manufactured by TencorInstruments Japan Co., Ltd.).

TABLE B4 Properties of various magnesium oxide films Form of Averagefine particle Film Film Sample particles diameter Adhesion strengthTransparency Crack thickness Ex. B1 Particulate  30 nm Good Good GoodFree 0.2 μm Comp. Ex. B1 — — Failure Failure Failure — — Comp. Ex. B2Cubic 800 nm Good Good Failure Free 3.0 μm Comp. Ex. B3 Flaky 200 nmGood Good Good Free 0.5 μm

In Comparative Examples B1, since the wetting property of sol solutionwas poor, conditions such as coating speed were varied. However, a wetcoating could not be evenly formed on the dielectric layer 5. This ledto the evaluation results given in Table B4. For comparative Example B2,the film thickness was 3 μm, i.e., larger than that in the othercomparative examples and the example, and the film was whitish and hadpoor transparency. Comparative Example B3 had no significant problem.

Then, for the opposed discharge alternating current type plasma displaysexcept for those in Comparative Examples B1 wherein no protective layercould be formed, an alternating current pulse having a waveform of adrive frequency 15 kHz and a duty ratio of 23% was used to measure thefiring voltage Vf and the minimum maintaining voltage Vsm.

After setting at the firing voltage, continuous discharge was performedfor 60 min. Thereafter, the minimum maintaining voltage was measured andcompared with the initial minimum maintaining voltage to investigate achange in voltage with the elapse of time. Thus, the life property ofthe panels was evaluated. The results are summarized in Table B5.

TABLE B5 PDP properties of various magnesium oxide films Minimum Firingmaintaining Sample voltage voltage Life Ex. B1 155 V 110 V Good Comp.Ex. B2 240 V 175 V Somewhat poor Comp. Ex. B3 160 V 105 V Good

As is apparent from Table B5, for the panel prepared in the example ofthe present invention, the firing voltage was 155 V, the minimummaintaining voltage was 110 V, and no variation in the minimummaintaining voltage was observed, that is, no problem was found in thelife property. For the panels prepared in the comparative examples, thepanel prepared in Comparative Example B2 had a firing voltage of 240 Vand a minimum maintaining voltage of 175 V which were higher than thosein the panel of the example of the present invention, necessitating ahigher panel drive voltage. Regarding the life property, 30 min. aftersetting at the firing voltage, an increase in the minimum maintainingvoltage was observed. For Comparative Example B3, the firing voltage was160 V, the minimum maintaining voltage was 105 V, and no variation inthe minimum maintaining voltage was observed, that is, no problem wasfound in the life property.

The results for the evaluation of the panel prepared in the example ofthe present invention, in comparison with the panels prepared in theother comparative examples, especially the panel prepared in ComparativeExample B3 by vacuum deposition which had achieved satisfactory resultsin the prior art, show that the panel developed functions inherent inthe magnesium oxide film and was satisfactory.

From the above results, it is apparent that the protective layer of amagnesium oxide film in the opposed discharge alternating current typeplasma display prepared in the example of the present invention exhibitssatisfactory properties in opposed discharge alternating current typeplasma displays.

The use of the sol solution and the film forming method according to thepresent invention, for example, in the construction of an alternatingcurrent type plasma display, can provide a protective layer of acontinuous film of magnesium oxide constituted by granular particleshaving a particle diameter of not more than 100 nm. Therefore, theprotective layer has high transparency, and the thickness of theprotective layer can be reduced. Further, since the film strength ishigh, there is no fear of the dielectric layer being exposed bycracking. Therefore, a lowering in drive voltage can be accelerated,enabling a reduction in cost of drive circuit, which in turn leads to areduction in cost of the body of the plasma display.

Further, since the protective layer can be formed by coating a solsolution, the film can be formed in a larger area at a lower cost ascompared with film formation using a thin film process, realizing theproduction of a plasma display having a large area (for example, about40 in. in terms of diagonal distance) at a low cost.

Furthermore, in the sol solution according to the present invention,since an organic compound having at least one hydroxyl group or anorganic solvent containing at least one compound of the above type isused as the solvent, the fine particles of magnesium hydroxide are lesslikely to agglomerate, offering a sol solution with fine particles ofmagnesium hydroxide stably dispersed for a long period of time.Therefore, a magnesium oxide film prepared by coating the sol solutionand sintering the coating is constituted by refined particles, improvingthe transparency of the resultant protective layer.

Further, according to the film forming method of the present invention,since an organic solvent is used as the solvent for the sol solution,upon sintering, no organic matter is left as a residue in the magnesiumoxide film, realizing the preparation of a protective layer constitutingof pure magnesium oxide.

In general, when the thickness of the protective layer in an alternatingcurrent type plasma display is thick, for example, as thickness as about10 μm, the wall charge effect, a source of memory function, which is oneof the important properties of the alternating current type plasmadisplay, is deteriorated, necessitating a higher drive voltage.Consequently, specifications for transistors used in the drive circuitshould cope with the use of high voltage. For this reason, the formationof a protective layer having a thickness of not more than 2 μm is themost important necessary and sufficient condition from the practicalviewpoint. The sol solution and the method for film formation accordingto the present invention, the protective layer having a thickness of notmore than 2 μm can be satisfactorily formed.

EXAMPLE C1 (Preparation of sol solution)

(1) Dehydration of Magnesium Acetate Tetrahydrate

A commercially available magnesium acetate tetrahydrate was dried at120° C. for 5 hr. in a vacuum dryer to prepare anhydrous magnesiumacetate from which the hydrate had been completely removed.

(2) Preparation of Sol Solution Containing Dispersed Colloidal Particlesof Magnesium Acetate

3 parts of the anhydrous magnesium acetate particles was added to anaqueous solution of 0.1 part of hydroxypropylmethyl cellulose in 97parts of water, and the mixture was stirred at room temperature for 3hr.

In the resultant sol solution with colloidal particles of magnesiumacetate dispersed in water, the diameter of the colloidal particles was120 nm. The sol solution was allowed to stand at room temperature forone month. As a result, it was stable, and settling of magnesium acetatewas not observed.

EXAMPLE C2 (Formation of protective layer on alternating current typePDP)

The formation of a protective layer, for an alternating current typePDP, using the sol solution prepared in Example C1, applied to a display(a panel), of a color television will be described. Embodiments of theconstruction of such a panel are shown in FIGS. 1 and 2.

As shown in FIG. 1 (plane discharge system), the display comprises afront substrate 1 and a back substrate 2 disposed opposite to each otherwith a gas discharge space 3 sandwiched between these two substrates.The substrates 1 and 2 are formed of a glass plate having apredetermined thickness. A pair of electrodes 4 of an electrode X 4 aand an electrode Y 4 b are provided on the substrate 1 in its sidefacing the substrate 2, and a dielectric layer 5 is formed on thesubstrate 1 so as to cover the electrodes. The dielectric layer 5 iscovered with a protective layer 6. In general, the thickness of theprotective layer 6 should be not more than 2 μm. An address electrode 7is formed on the substrate 2 in its side facing the substrate 1, and aphosphor is provided on the address electrode.

FIG. 2 shows another embodiment (counter electrode system) of theconstruction of the panel. In this embodiment, an electrode X 4 a isformed on a front substrate 1 in its side facing a back substrate 2, theelectrode X 4 a is covered with a dielectric layer 5, and the dielectriclayer 5 is covered with a protective layer 6. On the other hand, anelectrode Y 4 b, a dielectric layer 5, a protective layer 6, and abarrier 8 and a phosphor 9 are provided on the substrate 2 in its sidefacing the substrate 1.

The protective layer 6 in the FIGS. 1 and 2 was formed as follows.

The sol solution prepared in Example 1 was coated on the surface of thedielectric layer 5 by dip coating, a kind of coating printing, to athickness of 2 μm in terms of thickness after drying in the air. Afterthe formation of the coating, the coating was dried at 120° C. for onehr. and then heat-treated (sintered) at 500° C. for 2 hr. It was furtherbaked to evaporate water as water vapor.

The resultant magnesium oxide film was completely transparent and firmlyadhered the dielectric layer. Observation of the magnesium oxide filmunder a scanning electron microscope has revealed that particles in thesurface of the magnesium oxide film are even and fine and constitute adense film and the film thickness was not more than 2 μm.

COMPARATIVE EXAMPLE C1

A protective film of magnesium oxide was formed in the same manner as inExample C2, except that aqueous ammonia was added to an aqueous solutionof magnesium acetate tetrahydrate and a protective film of magnesiumoxide was formed using a liquid containing colloidal particles ofmagnesium hydroxide produced by hydrolysis. The magnesium oxide filmthus formed had uniformity much inferior to the film formed in ExampleC2 and unsatisfactory adhesion to the dielectric layer.

According to the present invention, for example, it is possible to forma magnesium oxide-containing layer, with a thickness of not more than 2μm, having excellent adhesion, to a substrate, such as a dielectriclayer, which has been unattainable by the use of a printing paste usedin the prior art for the formation of a magnesium oxide-containinglayer, enabling the production of an alternating current type PDP havinglow firing voltage and drive voltage (voltage consumption).

Further, according to the present invention, since a coating method maybe used, a film, such as a protective layer, having a large area can beformed at a low cost, realizing the production of a display for a largecolor television (for example, 40 in. in terms of diagonal distance).

What is claimed is:
 1. An alternating current type plasma displaycomprising: a back substrate and a front substrate disposed opposite toeach other; a gas discharge space sandwiched between the front and backsubstrates; a pair of electrodes provided on at least one of thesubstrates and covered with a dielectric layer; and a protective layerprovided on the dielectric layer, the protective layer being constitutedby a continuous film formed of fine particles of magnesium oxide havinga diameter of 5 to 100 nm.